You don't have to remove anything from the computer. You just have to make sure that your account is setup on the BES and that your Desktop Redirector is no longer running on the computer. You can then sync over wire or do an Enterprise Activation.

The best way to handle this situation is actually to modify Desktop Manager thorugh Add/Remove programs in your control panel and modify it to use BES.
After doing that go into your blackberry and go Options -> Service Book and delete any desktop service books listed there, after that is completed simply make sure that you have been provisioned on the BES perform an enterprise activation (Options -> Enterprise Activation).